FIG. 1 shows a simplified block diagram of a multirate digital filter 10 to digitally filter an input signal 12, for example in the form of a 96 kHz datastream as used in audio signal processing. While a filter structure such as shown in FIG. 1 may in general be prior art, such a filter structure that uses filter coefficients determined according to one or more aspects of the invention is not prior art. The multirate filter 10 includes a plurality of digital FIR filter devices 16_0, 16_1, 16_2, . . . , 16_n, where the number of FIR filter devices in the plurality is n. The FIR filter device 16_0 in the example shown in FIG. 1 includes a 57 tap FIR filter, whereas the remaining n−1 FIR filter devices 16_1, . . . , 16_n include 29 tap FIR filters. Other embodiments that use FIR filters may have filters with a different number of taps.
For each of the stages associated with the FIR filter devices 16_1 to 16_n, a rate decimation by factor of 2 is performed prior to filtering by the respective FIR filter devices 16_1, . . . , 16_n. A rate multiplication by a factor of 2 also is performed in each of the stages on the output side, with the outputs being summed over the various stages to generate the summed output 18 of the multirate filter 10. The summers sum signals samples at the same rate.
In multirate filter designs such as the ones shown in FIG. 1, changes of the sample rates between the different stages are exploited to increase computational efficiency while achieving a desired filter response over a relatively wide frequency range.
Each sample rate change is achieved, for example, by using a low-pass anti-alias filter.
There is a need in the art for a method of determining filter coefficients for the filter stages of a multirate filter, e.g., a multi-stage filter structure such as shown in FIG. 1, in order to achieve a desired filter response. Achieving a desired filter response may be achieved, for example, by a method of determining coefficients that achieve in a filter an actual filter response that is a close match to the desired response.